As is known, spinal fusions may be indicated when there is a deformity of the spine, instability of the spine, a damaged intervertebral disk, trauma, tumor, pain, infection and/or degeneration of the spine. A number of posterior spinal fixation methods are available to instrument the spine for stabilization purposes and to allow for a fusion to occur. Commonly used devices include transpedicle and facet screws. Many transpedicle screw systems that stabilize the spine are dependent upon the pullout strength of a non-curved transpedicle screw for fixation stability. Unfortunately, transpedicular screws usually fail either by breaking at the connection between the transpedicular screw and the assembly to which it is attached, or by failure of the transpedicular screw to bone interface. A transpedicular screw can fail within the bone for multiple reasons, including osteoporotic nature of the bone, lack of fill within the pedicle, infection and/or pseudoarthrosis of the fusion mass. A straight screw is reliant upon the shear resistance that is created by the threads of the screw and the radial force created by the size of the screw relative to the inner diameter of the pedicle. In order to achieve better fixation, larger transpedicle screws are sometimes used, which may result in a fracture or perforation of the pedicle resulting in nerve root or spinal cord injury. At the upper end of instrumented construct, most transpedicle screws have to violate the normal facet joint, which may lead to an increase risk of degeneration of the adjacent disk. At the upper end of the instrumented construct, traditional transpedicle screw technique may violate the bordering facet joint creating additional mechanical driving force for adjacent segment disease and the potential for further surgery over time in that adjacent segment.
Facet screws are also commonly known and rely on the pullout strength of the screw to achieve stability. Like transpedicular screws, facet screws are typically straight and rely upon the thread geometry and thickness of the screw to increase the amount of force needed for the screw-bone construct to fail. This is a disadvantage in osteoporotic, pathologic, or low density bone where there is less for the screws to anchor to and a decrease in the pullout strength of the screw, especially in shear. Furthermore, when inserting a transpedicle or transfacet screw, a straight screw can penetrate the side or front of the vertebral body due to the angle of insertion. The transpedicle screw may also penetrate the pedicle wall possibly injuring the exiting nerve root. This penetration may significantly risk neurological or vascular structures.
On the posterior end of the pedicle screw, a linkage that allows connection of the pedicle screw to a transverse rod and is often a tulip is provided that extends vertically along the spine. The vertical rod is attached to the tulip of each of the various screws, connecting several vertebral bodies together. The rod preferably stabilizes the vertebral canal and the spine, often to achieve a fusion of the spine for a variety of reasons, including back pain, neurological problems, fracture, curvature of the spine, degeneration of the spine and/or tumor. Thus, the screws and the tulip and rod are a mechanism often utilized for keeping the vertebral bodies in a generally fixed position relative to each other while the vertebral bodies are fused together by a bone graft. Spinal fixation systems form a scaffold with multiple fixation points in the segmental spinal anatomy. This created vertical assembly, sometimes with transverse linking, seeks to maintain spinal architecture under physiological loading while biological materials form a final union between segments within the span of this scaffold.
An additional problem with current devices is safety. The spinal cord travels behind the vertebral body. Nerves extend off the spinal cord between each of the various bodies and extend under the pedicle, or under each of the pedicles. In the thoracic spine, for instance, the spinal cord occupies a very small space within the confine of the vertebral bodies. Therefore, a very small margin of error exists anatomically when forming the scaffold function of spinal instrumentation with pedicle screws fixed to each thoracic vertebral level. In the lumbar spine, the nerves float in a spinal fluid sac exiting at each spinal level to perform critical motor and /or sensory functions in the lower extremities and for normal control and function of the bowel and bladder. Since the screw must fit closely to the diameter of the pedicle to occupy its volume and thus have biomechanical strength, the margin of error is minimal. In other words, the technique of delivery of pedicle screws in the thoracic and lumbar spine carries inherent risk to the geometry of design of the pedicle and screw and the vector of placement into the vertebra. When a physician performs spinal surgery in the lumbar spine, a pedicle screw can perforate or extend through the side of the pedicle and touch the nerve so as to cause nerve damage. This particular problem often occurs in the medial and/or inferior quadrant of the pedicle, where the nerve route most closely contacts the pedicle. When a thoracic pedicle screw is placed, medial deviation can impale the spinal cord and imprecise placement may strike the anterior vascular structures with disastrous results.
Accordingly, what is needed is a device and method for safely stabilizing adjacent vertebra in the mammalian spine.